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We report on the development of a new family of magnetic field sensors with exceptionally low magnetic field noise, as low as 0.3 fT Hz −1/2 . Beside this, they exhibit high usable voltage swings of more than 150 μV pp and tolerable background fields during cool-down of up to 6.5 mT. In operation mode they recover completely from magnetization pulses of up to 76 mT, which makes them well suited for applications such as low-field magnetic resonance imaging.With respect to their easy and reliable use as well as their field resolution in the sub-fT Hz −1/2 range, the presented SQUID sensors are adequate for many applications, such as in geophysics or in biomagnetism.

We report on a technology for the fabrication of sub-micrometer sized cross-type Josephson tunnel junctions in niobium technology. We present the fabrication scheme and properties of cross-type junctions with linear dimensions from 10 down to 0.6 µm. Sidewall passivation of the junctions is achieved by anodization as well as by planarizing the junctions with SiO in a self-aligned deposition step. The measured ratio of the sub-gap resistance to the normal resistance is about 35. Because of their low sub-gap current and low parasitic capacitance such junctions are well suited for applications like high resolution SQUIDs.

We have developed highly sensitive SQUID-based magnetometers and gradiometers, fabricated in a standard Nb/AlOX/Nb technology. The SQUID itself is designed as a current sensor having an input coil and a feedback coil. The number of turns of the input coil can be adjusted to ensure optimal coupling to the pickup loops with an inductance in the range from 5 nH to 300 nH. Several types of planar pickup loop configurations have been realized. The magnetometer has a pickup loop with a size of 1 cm × 1 cm. With a typical white noise level better than 4 µ0 Hz-1/2 and an effective area of 2.6 mm2 a field resolution of 3.2 fT Hz-1/2 results for the magnetometer. Two loops connected in series with an area of 2 cm × 2 cm each and a baseline of 4 cm were used in the gradiometer. We measured a 7.5 mm2 effective area for each loop and a field gradient resolution of 36 fT m-1 Hz-1/2 corresponding to a field resolution in the loop of 1.6 fT Hz-1/2.

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